The invention relates to an arrangement for measuring a relative linear position between a sensor and a magnetic body, whose position relative to each other can be changed linearly in a predefined direction.
Such an arrangement is known from DE-OS 42 33 331. The arrangement disclosed there includes a sensor with two sensor elements whose signals are evaluated by means of measuring bridges. The relative linear position can be determined to a magnetic body that has a periodic multi-pole structure. The sensors measure in a unidimensional manner i.e. they measure in a certain measuring direction the strength of the occurring magnetic field. When there is a change of position, the sensor detects the number of the magnetic poles from the signal of one measuring element, the direction of movement from the signals of the two measuring elements and, in addition, the analog position within the range of one magnetic pole from the signals of the two measuring elements. The arrangement can process a sine/cosine signal from the two sensors only when there is a really optimum match between sensor and encoder. Inaccuracies of the magnetic field, i.e. deviations from the ideal sine shape which generally occur in practice, phase errors and magnetization errors of the magnetic structure, and different sensitivities of the two sensor elements have a considerable influence on the result produced by the arrangement. Thus, the arrangement produces the desired result only under ideal conditions, which can hardly be achieved in practice, which desired result is the exact indication of the linear position between sensor and magnetic body.
It is an object of the invention to further develop the arrangement defined in the opening paragraph in that it produces the best possible, that is, most accurate, result of the linear position at little expenditure and in that it is highly independent of the variations of field strength and temperature and distance tolerances.
This object is achieved according to the invention in that the sensor is arranged as a magnetoresistive angle sensor and in that a magnetic strip is arranged as the magnetic body, which strip has a pair of magnetic poles and magnetic field lines running over its length at varying angles, so that the angle of the magnetic field penetrating the magnetoresistive angle sensor depends on the relative position of the magnetoresistive angle sensor to the magnetic strip and the relative position can be computed from the output signal of the magnetoresistive angle sensor.
The magnetoresistive angle sensor exclusively works as an angle sensor, that is, the resistance of at least one sensor element in the sensor changes with the direction of the magnetic field running through the sensor. The magnetoresistive angle sensor is therefore insensitive to variations of the field strength of the magnetic field running through it. This again leads to the fact that the measuring result is independent of the strength of the magnetic field of the magnetic body, of the distance between sensor and magnetic body and independent of the temperature. The magnetic strip relative to which the position of the sensor can be determined, includes a pair of magnetic poles. The magnetization over the length of the magnetic strip is then provided such that the magnetic field lines viewed over the length of the magnetic strip show varying angles. In consequence, the relative position of the sensor to the magnetic body can be computed from the angle of the field lines passing through the sensor.
As a result of an angle measurement that is relatively insensitive to disturbances caused by external influences and variations of the magnetic field strength, this simple-structure arrangement provides a really accurate relative linear position between sensor and magnetic body. It is not necessary for the arrangement, as it is for many state-of-the-art arrangements, to have a supporting magnet for calibrating the sensor. Furthermore, since the magnetoresistive angle sensor measures only angles and not field strengths, the arrangement is insensitive to external influences, which often occur under special mounting conditions, for example, in vehicles.
In accordance with an embodiment of the invention, the magnetic strip is advantageously arranged as a flat strip in which the magnetoresistive angle sensor is positioned on a longitudinal side of the magnetic strip, so that the field strength running through it is maximum and the sensor measures only angles and not field strengths.
Advantageously, the magnetoresistive angle sensor as arranged in a further embodiment has two sensor elements, which are concentric to each other and turned through 45xc2x0 to each other. This achieves the advantage that an angle range of xc2x190xc2x0 of the magnetic field can be measured and, in addition, this range can be reached with a high resolution and accuracy. This provides that the measuring range of the arrangement can be expanded to relatively large mechanical ranges, for example, to a measuring range of 100 mm.
Advantageously, known evaluation circuits for magnetoresistive angle sensors can be used for the arrangement, which evaluation circuits, as is provided in accordance with a further embodiment may be arranged as integrated circuits which use the CORDIC algorithm as a simple method for computing an angle.
It may perhaps be desirable, depending on the purpose of use, to measure the linear position not with constant accuracy over the entire measuring range, but to achieve an increased accuracy of the measurement of the linear position in certain areas. For this purpose, as provided in an additional embodiment, the characteristic curve of the magnetic field over the length of the magnetic strip has a non-linear pattern. This means that in some areas the change of angle of the magnetic field per unit of length of the magnetic strip shows a stronger variation than in other areas. Thus, in the areas in which the change of angle of the magnetic field per unit of length of the magnetic strip shows a stronger variation, an increased measuring accuracy can be achieved.